This applies preferably to a situation in which the engine has stopped and the motor vehicle ignition circuit has been switched off, and therefore not to a situation in which the engine will very shortly be restarted, which is the case for example on a stoppage imposed by an automatic stop-start system of the engine, such a short stoppage not necessarily being suitable for implementation of the method according to the present invention.
In the context of the invention, the internal combustion engine of the motor vehicle is preferably a petrol engine, but this is not limitative.
For one or each cylinder of the engine, an injector is provided which is supplied with fuel by an injection rail which is pressurized during operation. The injection rail remains pressurized for a certain period following the stoppage of the engine when the vehicle's ignition circuit has been switched off, which leads to fuel leakages through the injector.
Conventionally, a fuel injection system in an internal combustion engine comprises one or more injectors, a fuel supply circuit called a low-pressure circuit, and an injection circuit called a medium- or high-pressure circuit with a high-pressure pump, opening into an injection rail which supplies the injector or injectors and distributes fuel into the injector or injectors. The system also comprises a return circuit for the unused fuel. The low-pressure circuit may comprise a fuel filter and is connected upstream to a fuel tank.
When the engine is switched off by turning an ignition key, the pressure of the fuel in the high-pressure circuit of a direct-injection engine is relieved via a discharge valve. This pressure fall may also be achieved via internal leakages of the high-pressure pump and internal leakages of the injector or injectors when the configuration of the injector or injectors allows, and external leakages of the injector or injectors.
Therefore when the system has no discharge valve or internal leakages in each injector, the pressure fall in the injection rail is linked solely to the internal leakages of the pump and the external leakages of each injector into the combustion chamber of the engine. The time for pressure reduction is very variable depending on characteristics, and may be very long, up to 24 hours. The external leakages from each injector vary with pressure, and hence vary over the period for which the engine is stopped, until they disappear completely when the pressure falls to atmospheric pressure. The accumulated fuel from these external leakages of the injector into the engine combustion chamber, which is itself directly connected to atmosphere when the inlet or exhaust valves are open, generates pollution from unburned hydrocarbons when the engine is next started.
This is illustrated in FIG. 1 for a petrol engine. The pressure in the rail is 200 bar when the vehicle has stopped, and falls progressively over time to return close to atmospheric pressure after 40 minutes. It is assumed that approximately 1 μl of fuel is lost for each minute of cooling of the engine, which gives a loss of 40 μl for each injector if the pressure returns to atmospheric pressure after 40 minutes, which is not an insignificant amount.
Document WO-A-2012/072607 describes a method for estimating the quantity of leaked fuel which flows from an injector during the stoppage period of a motor vehicle. The method comprises steps consisting of measuring a first start-up index, determining a first quantity of fuel injected during the first start-up, measuring a second start-up index, determining a second quantity of fuel injected during the second start-up, and estimating the quantity of fuel based on the first start-up index measured, the first determined quantity of injected fuel, the second start-up index measured and the second determined quantity of injected fuel.
However, although this document acknowledges the problem of fuel leakage after stoppage of the engine, no solution is given for reducing this leakage, the method described in the document serving merely to estimate the fuel leakage.
Document FR-A-3 014 492 describes a method for regulating the fuel temperature in a fuel injection system for an internal combustion engine of a motor vehicle, the injection system comprising an injector associated with an injection rail connected to a high-pressure injection circuit, in which both cooling and heating of the fuel may take place. The fuel is cooled as an alternative or in addition to further cooling from an air-conditioning system present in the motor vehicle, the air-conditioning system having a branch duct at least partially surrounding the low-pressure fuel circuit.
The regulation method described in this document is not concerned with the problem of leaks from an injector during a period following stoppage of the engine, and does not therefore offer a solution to such a problem. Furthermore, although cooling takes place on the low-pressure portion of the injection circuit, this has no effect on the high-pressure portion and does not therefore act to reduce leakage.
The problem on which the present invention is based is to reduce the leakage from a fuel injector in an internal combustion engine of a motor vehicle when the engine has stopped and remains in a prolonged stoppage, the ignition circuit of the motor vehicle having been switched off.